Infrared detecting materials

ABSTRACT

Infrared detector material is formed by the epitaxial growth of a single crystal alloy of the two III-V compounds InAs and InSb on an InAs substrate. In the method of such growth, a liquid solution is prepared with excess indium solvent, InSb, and sufficient InAs to saturate the indium at 500* C. The InAs substrate, oriented in the III direction is immersed in the solution, and the substrate and the solution are brought to equilibrium at approximately 500* C. Slowly lowering the solution temperature causes a single crystal to be epitaxially grown on the substrate as a solid homogeneous InAs-InSb solution. Composition of the crystal is a function of solution composition and may be controlled by dissolving selected quantities of InSb in the solution.

United States Patent [72] Inventors Jerry W. Moody 3,351,502 11/1967Rediker 148/ 177 Worthington, Ohio; 3,462,323 8/1969 Groves 148/175 [2]]A l N J. Reid, Syosset, N.Y. 3,463,680 8/1969 Melngailis et a1 148/172o. [22] 6, 1970 Prirnary Examiner-L. Dewayne Rutledge [45] Patented Be28 1971 Assistant Exammer-E. L. Wetse [73] Assign Avco corponuonAttorneys-Charles M. Hogan, [mm P. Garfinkle and Jerome Cincinnati, OhioOriginal application June 5, 1968, Ser. No. 734,715, now Patent No.3,558,373, dated Jan. 26, 1971. Divided and this application Apr. 6,1970, Ser'. No. 31,063

ABSTRACT: Infrared detector material is formed by the epitaxial growthof a single crystal alloy of the two Ill-V com- [54] gf z g f zgMATERIALS pounds lnAs and lnSb on an lnAs substrate. In the method of gsuch growth, a liquid solution is prepared with excess indium 29/194,solvent, lnSb, and sufficient InAs to saturate the indium at 148/ 1.6500 C. The lnAs substrate, oriented in the 111 direction is imllll-B3211 15/00 mersed in the solution, and the substrate and the solutionare [50] Field of Search 29/ l 94; brought to equilibrium atapproximately 500 C. Slowly lower- 148/ 1.6, 171; 75/134 T ing thesolution temperature causes a single crystal to be epitaxially grown onthe substrate as a solid homogeneous [56] Rehnnm cited lnAs-lnSbsolution. Composition of the crystal is a function of UNITED STATESPATENTS solution composition and may be controlled by dissolving3,312,570 4/]967 Ruehrwein selected quantities of InSb in the solution.

PATENTED UEEZB l97| FIG I ANT IMONY HA5 ARSENlC INVENTOR.

vgivuvvu AYYYAYAVAVAVA INDIUM WWW/ JERRY w MOODY BY E J. REID M ATTORNEY1 INFRAREU verse-nus MATERIALS This is a division of application Ser.No. 734,715 filed June 5, l968hw US. Pat. No. 3,558,373, dated Jan. 26,I971.

BACKGROUND Our invention relates to' infrared "detector materialsinvolving semiconductor crystals; anthrnore particularly relates toepitaxially grownsingle" crystal alloys of III-V compounds: In thisconnection, it may be' 'pointed out that by'groupll l elements'we meanaluminum gallium and indium; and by group V elements, we meanphosphorous, arsenic, and antimony. Various compounds of group lll'andgroup V elements have been found to have properties which, for someapplications, aresupe'rior to those of the-groupIV semiconductormaterials. They offera wider range of energy gaps; Several methods havebecome known for the preparation of various single III-V compoundcrystals; Homogeneous alloys by chemical vapor deposition have been madeof certain gallium, aluminum, indium, phosphides-and arsenides, butattempts to make'other pseudobinai'y III-V alloys (e.g'., lnAs and InSballoys) by chemical vapor deposition haveno't met withsucce'ss;

After the investigation of lIl'-V compounds was begun, it wassuggested-that alloysof 'these compounds could be made, therebyextending-the range of'pr'operties' of the Ill-V compounds. Ofparticular interest to us'were' alloys of' indium arsenic and indiumantim'onid'e. These have been found to' have an energy gap rangingfrom'ab out 0.1 ev. or less-to 0.45 ev., depending"oncomposition" andtemperature, making them especially well suited forinfrared detection.

At least three methods have been used to make alloys of lnAs and InSb.These have usually resulted in polycrystalline alloys, or at bes'tinnonhom'ogeneoussingle crystal alloys; The properties of lnAs-lnSballoys" have been deterrnin'ed from such polycrystalline ornonhomogeneous single crystal alloys; Although the art'has forseveralyearsappreciated 'the desirability of single crystalhoniog'eneoussolidsolutions of Ill-V compounds; and althoog'hseveral attempts h'avebeen made to producesuch crystals; to our knowledge none of 'theattempts madep'riorto our'in'ventioh have been successful.

The earliest attempt toproduce homogeneous ingots was the annealing offinecompressed powders of lnAs and InSb. Although this method didproduce'ingots'with homogeneous regions, the ingot itself was nothomogeneous. This methodtakes anywherefrom several weeks to severalmonths and results in polycrystalline ingots.-

Zone recrystallization, similar to zone refining, has also been used.However, this method, while producing ingots with homogeneous regions;producespolycrystalline ingots;

Directional freezinghas' also been used, but it too producespolycrystallineingots.

There is needfor an infrared detector material utilizing a singlecrystal homogenous alloy of Ill-V compounds.

There is a need for a rr'iethod'w'hich can be used 'to produce singlecrystal homogenousalloys of III-V compounds.

There is a needfor a composition which can be used to produce singlecrystal homogenous alloys of III-V compounds.

It is therefore an object of our invention to provide infrared detectormaterial utilizing a single crystal homogenous alloy of III-Vcompounds.-

It is a further object" of our invention to provideepitaxially' grownhomogeneous alloysof Ill-V compounds on a suitable substrate.

A further more specific object-of our invention is to'provide a methodand a composition fertile-epitaxial'growthof a solid homogeneoussolution of lnA's and InSb on' a substrate of either lnAs or InSb;

Further objects and features of our invention will be apparent from thefollowing specification and'claims when considered in connection withthe accompanying drawings illustrating several embodiments of ourinvention;

SUMMARY OF THE INVENTION The invention involves, in one aspect, an'ew'infi'ared'detector structure; In anotheraspect; it involves amethodfor producingasingle crystal alloy of two lII-V- compounds; such"as for example lnAsand InSb, the'm'ethod'comprising; (a) melting the twoIll- V compounds and'a'su'itable solvent, such as excess indium, in acrucible; (b eff ctirig the saturation of thesolvent with one ofthecomp'ounds, such as for example lnAs; (c) immersinga suitableprepared 'substratein the solution, the substrate comprising a crystalhaving-a lattice structure and spacing similar to that of the III Vcompounds'dis solved in the solvent, such asfor example an' lnAssubstrate; (d) effecting growth of an alloycrystal on the substrate-bylowering/the temperature of the solution; (e) 'andremoving the substrateandthe'grown alloy crystal from the solution; wherein said compoundscrystallize onthe substrate to form a single crystal with improvedhomogeneity.

The invention also involves a composition which is a'solution for useinthe epitaxial growth of 'crystalscomprisin'g: (a) a solvent such as,for example, indium; (b) 'a'firstlll-V' com-- pound, such as for examplelnAs; dissolved in the solvent; and

(c) a second lll-V compound, such as for exam'plelriSb, dis solved inthe solvent. The solution is particularly useful when an excess indiumsolvent is saturated in either th m; or the InSb.

DESCRIPTIONOF THE VIEWS FIG. 1'- is a view in front elevation'ofanapparatususeful' with our invention to produce crystals, the apparatusbeing shown with a segment removed to expose, in vertical section, theinside of the-apparatus;

FIG. 2 is a triangular coordinate diagram illustratingrthe DETAILEDDESCRIPTION Apparatus used by us is similar to a Czochralslti crystalpulling apparatus and is illustrated in-FIG. 1. Generally, it comprisesa suitable cylindricalcasing l0'witha sealable top 12. The casing has apedestal 14 to supporta-graphite crucible 16.

A substrate holder 18, which-can be amodified'Czochralslti pulling rod,extends through the top 12- and'is longitudinally (i.e., vertically)movable. The rod, however, should not be a good heat conductor since wedo not want'heat'loss through the rod. An RF or resistance heating coil20 surrounds the casing 10 for heating the crucibleand its'con'tents'.

Suitable ternperature-sensing devices, such as thermocouple transducers,may be contained bytwo thermocouple tubes such as 2211 and 22b. One suchthermocouple tube 2 2:; positioned within the substrate holder 18isconnected by awire 24 to a temperature-indicating device (not shown)and senses-the temperature of the crucible contents;

In general, out method begins by obtaininga=soliition oftwo selectedIll-V compounds preferably having acommon group III element. This" isdone by dissolving the compounds in a suitable solvent which we preferto be an excess of the common group III element. We prefer to dissolve asutficient quantity of one of the III-V compoundssothatwe can saturatethe solution in that compound" at a'selected-equilibrium temperature.The quantity of the other lll -V compound is selected inorder to resultin a desired crystalcomposition.

Thus the two Ill-V compounds are first melted together with the solventto obtain the desired solution, and the solution is homogenized. Asubstrate is then immersed in the solution and equilibrated with it atan equilibrium temperature. The substrate should be a material which hasthe same lattice structure as each of the III-V compounds and as nearlyas practical the same lattice spacing. We prefer to use a substrate ofone of the two III-V compounds of the solution, and in particular of theIII-V compound in which the solution is saturated.

Such a substrate when immersed in and equilibrated with the solutionprovides a favorable site for precipitation. As the solution then iscooled, a single crystal alloy is epitaxially grown on the substrate.The composition of the epitaxial layer is a function of the solutioncomposition. After growth, the substrate is withdrawn, treated, andtested in ways familiar to those skilled in the art.

We have found it desirable to soak the substrate in the solution atequilibrium prior to initiating growth. This is especially helpful whenwe plan to cool at a fast rate such as approximately 200 C. per hour. Insuch case, we have "soaked" the substrate for 2 hours before, and thesubstrate and grown crystal 2 hours after, growth.

To grow lnAs-lnSb alloy crystals according to the preferred embodimentof our invention, we first obtain a solution of InAs and lnSb dissolvedin indium. The solution is obtained by melting together suitable amountsof lnAs, lnSb and excess indium in the crucible 16. Heat is suppliedfrom the heater 20. Of course, prior to melting, the substrate holder 18is maintained out of the crucible 16 so that a suitably preparedsubstrate can be positioned in a lateral slot in the holder 18 at theraised position of 30a.

After the indium, lnAs, and lnSb are melted and the solution 32 in thecrucible 16 is homogenized, the substrate holder 18 is lowered, and thesubstrate is immersed in the solution 32 at the lowered position of 30b.The substrate and the solution are equilibrated at a temperature whichis between the melting point of indium (155 C.) and the melting point ofthe substrate which in the case of lnAs is 942 C. and in the case oflnSb is 525 C. We have at times used an equilibrium temperature of about500 C. If the solution is saturated in lnAs, we prefer to use an lnAssubstrate. As we cool the solution 32, a single crystal alloy of lnAsand InSb is obtained on the substrate by epitaxial growth.

FIG. 2 and FIG. 3 illustrate the compositions of some of the specificexamples we have performed and the crystals we have obtained. Thecoordinates are for mole percentages of indium, arsenic, and antimony.The lines 50 and 51 represent a range of liquid solution compositions,and the lines 52 and 53 represent the range of crystal compositions. Inthe examples illustrated in FIG. 2, the solutions were equilibrated atapproximately 500 C. before cooling and crystal growth was begun. In theexamples illustrated in FIG. 3, 400 C. was the approximate equilibriumtemperature.

In performing the examples, the indium, indium arsenide, and indiumantimonide used were substantially pure so that the compounds, substrateand alloy consisted essentially of the elements in the proportions andranges specified. However, it is obvious that other proportions may beused and that additional impurities could be either unintentionally orintentionally introduced.

The following examples illustrate the process and the composition of theinvention using particular materials, steps and conditions. It is to beunderstood that these examples are furnished by way of illustration andare not intended to be by way of limitation.

EXAMPLE I Indium, lnSb, and sufficient lnAs to saturate about 20 gramsof indium at 500 C. were melted together to obtain a solution,illustrated by the point 54 in FIG. 2, comprising in molepercentages-1,10 percent arsenic (1.36 percent lnAs), 18.12 percentantimony (22.44 percent lnSb), and 80.78 percent indium (72.20 percentexcess indium). This solution was equilibrated with an immersed lnAssubstrate oriented in the [III] direction, at 500 C., and was thencooled 20 C. at the natural cooling rate of the furnace of 197 C. perhour.

A single crystal epitaxial layer was grown 42 microns thick andcomprising 10 percent lnSb and percent lnAs as illustrated at point 56in FIG. 2.

EXAMPLE II Indium, lnSb and sufficient lnAs to saturate about 20 gramsof indium at 500 C. were melted together to obtain a solutioncomprising, in mole percentages 78.08 percent indium, 21.18 percentantimony, and 0.74 percent arsenic, as illustrated at point 58 in FIG.2. The solution was equilibrated with an immersed lnAs substrate,oriented in the [III] direction at 500 C., and was cooled 18 C. at arate of 5 C. per hour.

A single crystal epitaxial layer was grown comprising 24.0 percent InSband 76.0 percent lnAs, as illustrated at point 60 in FIG. 2.

EXAMPLE III Indium, lnSb and sufficient InAs were melted together toobtain a solution comprising, in mole percentages, 75.04 percent indium,24.30 percent antimony, and 0.66 percent arsenic. The solution wasequilibrated with an immersed lnAs substrate at 500 C. and was cooled 19C. at a rate of 1.6 C. per hour.

A single crystal epitaxial layer was grown comprising 50 percent lnSband 50 percent lnAs.

EXAMPLES IV-VI Further similar examples are illustrated in FIG. 2. Inall the examples I-VI, and lnAs substrate was equilibrated with asolution saturated in lnAs at 500 C.

EXAMPLE VII Indium, lnSb, and sufficient InAs to saturate about 20 gramsof indium at 400 C. were melted together to obtain a solutioncomprising, in mole percentages, 91.32 percent indium, 8.6 percentantimony, and 0.08 percent arsenic, as illustrated at point 62 in FIG.3. The solution was equilibrated with an immersed lnAs substrateoriented in the [111] direction at 400 C., and was cooled 40 C. at arate of 5C. per hour.

A single crystal epitaxial layer was grown 70 microns thick andcomprising 17.5 percent lnSb and 82.5 percent lnAs as illustrated atpoint 64 in FIG. 3.

EXAMPLES VIII and IX Further similar examples are illustrated in FIG. 3.In all the examples VII-IX, lnAs substrate was equilibrated with asolution saturated in lnAs at 400 C.

EXAMPLE X A solution of 20 grams of indium and 3 grams of lnSb washeated to 300 C., the 3 grams of lnSb saturating the solution in lnSb at300 C. An ingot of lnAs was positioned in the solutlon overnight toattempt to saturate the solution in lnAs. An lnSb substrate waspositioned in the solution and the solution temperature was lowered 23C. at a rate of 6 C. per hour. An epitaxial was formed on the substrate.

We prefer, for making infrared detectors, to obtain an alloy ofapproximately 50 percent or 60 percent lnSb because the alloy exhibitsan energy gap minimum of 0.1 ev. at approximately this composition.

It is to be understood that while the detailed drawings and specificexamples given describe preferred embodiments of our invention, they arefor the purposes of illustration only, that the apparatus of theinvention is not limited to the precise details and conditionsdisclosed, and that various changes may be made therein withoutdeparting from the spirit of the invention which is defined by thefollowing claims.

wherein the substrate consists essentially of indium antimonide.

4. An article of manufacture comprising a substrate consisting of awafer composed of a compound selected from the group consisting ofindium arsenide and indium antimonide having grown thereon a singlecrystal of a pseudobinary indium arsenide-indium antimonide allay.

t t i l Attesting Officer UNITED S'IATES PATENT OFFICE CERTIFICATE OFCORRECTIQN Patent No. 3,630,693 Dated December- 28, 1971 Inventor-(s)Jerry W. Moody and Francis J. Reid It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Line 6 of Abstract, 'III" should read "[III]".

Column 1, lines 27 -28, "arsenic" should read "'arse'nide".

1, line 69, "a method and a composition for the epitaxial growth of asolid should read "an epitaxially grown solid".

Column 3, line "1,10 percent" should read "1.10 percent".

Column 4, line 36, "and" shouldread "an".

4, line 64, "epitaxial was" should read epitaxial layer was".

4, line 67, "ev." should read "eV".

Signed and sealed. this 27th day of February 1973..

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK I Commissioner of Patents

2. A composition of matter according to claim 1, wherein the substrateconsists essentially of indium arsenide.
 3. A composition of matteraccording to claim 1, wherein the substrate consists essentially ofindium antimonide.
 4. An article of manufacture comprising a substrateconsisting of a wafer composed of a compound selected from the groupconsisting of indium arsenide and indium antimonide having grown thereona single crystal of a pseudobinary indium arsenide-indium antimonidealloy.